This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems which produce spatially compounded panoramic images.
A method of processing and displaying ultrasonic images which is presently commercially available is known as extended field of view or panoramic imaging. In a basic realtime imaging mode, an ultrasonic transducer produces realtime images of the area of the body immediately in front of the transducer aperture. When the transducer is moved to another location on the body, the images will be of the area in front of the transducer aperture at the new location. That is, as the transducer is moved along the body, the ultrasound system will continuously produce an image of the area currently in front of the transducer aperture at any given moment. Panoramic imaging produces a composite image covering all of the areas which were in front of the transducer aperture including its starting and ending locations and every location in between. Panoramic imaging was initially performed over twenty years ago by B-arm ultrasound scanners, which would send and receive scanlines as the transducer was moved. The spatial location of each scanline was tracked by position sensors in the B-arm and the scanlines then assembled to form a continuous image under the path of travel of the transducer. In today""s commercial systems the transducer used is an array transducer which continuously scans and acquires image frames. The need for the B-arm is obviated by correlating successive image frames on the basis of their common (overlapping) image content. When the array transducer is moved in a path such that successive image frames are overlapping and are co-planar there is generally sufficient correlation to align the image frames to each other. The aligned image frames are assembled together to form a continuous image of the tissue under the path of travel of the transducer. Panoramic images are produced in this manner by the methods and apparatus described in U.S. Pat. No. 5,782,766, for instance.
U.S. Pat. No. 5,538,004 shows another ultrasound display technique which utilizes frame to frame alignment. In this patent each new realtime image is aligned with a preceding one, then rotated as needed so that the viewer is always seeing the anatomy in the image from the same spatial reference, even as the transducer views the pathology from different orientations. The result is an image stabilizing effect. The author of the ""004 patent recognizes that the image alignment process could be used in other image improvement procedures such as compounding. In U.S. Pat. No. 5,566,674 this extension is made by acquiring images of the same anatomy in the body from different transducer positions as demonstrated in FIG. 1 of that patent. The images are aligned, then compounded to form a new image with reduced shadowing and speckle.
It is desirable to have an imaging technique which combines all of the favorable attributes of the above systems, that is, a panoramic image in which shadowing and speckle are reduced by compounding. The technique suggested by the ""004 and ""674 patents, acquiring images from different transducer positions, has numerous drawbacks, however. Many paths along which a transducer is moved for panoramic imaging are substantially linear, such as the leg when imaging the saphenous veins. While points in the image may be interrogated by different scanlines of the image frame from frame to frame, the apertures of the two scanlines are usually highly correlated and produce little if any compounding effect because the scanlines in the overlapping image areas are substantially parallel. In other areas of the body such as the abdomen, the torso is generally too large with too much attenuation to be able to view the same anatomy from both sides of the body. Thus the diversity in views of the anatomy needed for compounding often cannot be obtained by simply moving the transducer. Even if they can, the transducer displacement required to acquire them, the distortion caused by tissue deformation from the transducer, and the possibility that the two views will not be co-planar and hence not correlated all increase the likelihood that the panoramic imaging registration algorithm will fail. It is desirable to be able to produce spatially compounded panoramic images without these drawbacks.
In accordance with the principles of the present invention, spatially compounded panoramic ultrasound images are produced by acquiring images as an array transducer is translated in relation to an area to be scanned. As the transducer is moved scanlines are electronically steered in a plurality of look directions relative to the transducer. The electronically steered scanlines are combined to form spatially compounded images which are aligned and combined to form a spatially compounded panoramic image. In one embodiment spatially compounded images are initially formed, then aligned and combined to form the panoramic image. In another embodiment component image frames of different look directions are aligned and then combined to form the spatially compounded panoramic image in one combining step, which reduces blurring from the transducer motion. In the first approach, the spatially compounded images may be produced at the rate of once for each full sequence of look directions, once for each new look direction, or after a partial new sequence of look directions have been scanned.